1. Field of the Invention
This invention relates to load-bearing plates adapted to provide support for a generally planar surface, in general, and to mine roof bearing plates, in particular.
2. Description of the Prior Art
Attempts have been made in the past to provide mine roof plates made from a relatively thin material. In addition, there are many known examples of mine roof plates exhibiting high strength and rigidity under bolt-loading by means of deepdishing or excessive material thickness. However, in recent years there has been a trend in the mining industry toward the use of thinner plates in order to reduce cost, and minimize weight and installation problems.
Although there have been a number of prior designs for mine roof bearing plates, those which have been most readily accepted as meeting governmental standards have taken one of two primary design configurations: the so-called "donut-type" and the "bell-type". The donut-type is embossed in a manner such that a portion thereof, taken in cross section, looks very much like a donut; and the bell-type is embossed in a manner such that a cross section thereof looks very much like a flattened bell. In some instances, these plates are made from relatively more expensive high-tensile grade steel. Many variations of these two basic concepts exist in the prior art.
The United States Bureau of Mines, in existing specifications relating to bearing plates, indicates that one of the most important features of a bearing plate is its ability to resist axial deflection when it is fully bolt-loaded. To measure axial deflection, the government working with the American Society for Testing and Materials (ASTM) has developed a standard test. A mine bearing plate under test is placed on a standard test plate which contains a four-inch diameter opening. Specific preloads are applied to the bearing plate with the resultant axial deflection being measured. The four-inch diameter figure has been selected because this represents the crumbling loss in the bearing area with loads concentrated adjacent to a bolt hole drilled in a mine roof. Presently, to confirm that a bearing plate is of sufficient rigidity, the axial deflection of the plate, as it is bolt-loaded from 6,000 to 15,000 pounds, should be no greater than 0.120 inches. The deflection of the plate, as it is bolt-loaded from 6,000 to 20,000 pounds, should be no greater than 0.250 inches.
There is, thus, a need for a bearing plate made from a thinner and less expensive material than has heretofore been possible and which still meets or exceeds all government standards. The subject invention is directed toward filling that need.